This invention relates to a magnetic fluid sealing device for sealing the clearance between two members reciprocating relative to each other, with a magnetic fluid, and is applied, for example, to a light shielding device for shielding the light of the zoom lens portion of a camera or a dust sealing device of a precision instrument.
The magnetic fluid sealing device of this kind is interposed between the relatively reciprocating two members across two regions requiring a sealing property. And, the magnetic fluid sealing device seals the clearance between the two members by causing a magnetic fluid held on one of the two members to contact the surface of the other member (or the sliding contact member).
In this magnetic fluid sealing device, the surface of the sliding contact member to be contacted by the magnetic fluid may be subjected to the xe2x80x9cwetting-preventing treatmentxe2x80x9d for lowering the wettability with the magnetic fluid. In JP-A-2000-110948, for example, the sliding contact member is polished on its surface and is then subjected to a surface treatment for coating the shaft surface with a fluorochemical surface modification agent having an oil-repelling property. By this wetting-preventing treatment, the magnetic fluid is prevented at the time of relative reciprocations from sticking to and residing on the surface of the sliding contact member.
However, the case of the aforementioned related art has the following problems.
Generally, the magnetic fluid sealing device takes a longer time period in a stopped state where the two members stop at a specified position than in a moved state where the two members are relatively reciprocated.
Here, the oil-repelling property of the fluorochemical surface modification film formed on the surface of the other member degrades as the time period for contacting with the magnetic fluid is elongated.
Therefore, the fluorochemical surface modification film at the-portion contacted by the magnetic fluid when the magnetic fluid sealing device is in the stopped state is deteriorated so that the wetting with the magnetic fluid occurs.
When the wetting with the magnetic fluid thus occurs, the magnetic fluid sealing device cannot exhibit the sealing effect so that it terminates its lifetime.
In short, the deterioration of the fluorochemical surface modification film contacted by the magnetic fluid at the stopped time is a factor to determine the lifetime of the magnetic fluid sealing device of the conventional art.
Moreover, the magnetic fluid sealing device has a variety of application ranges and is demanded, depending upon the application range, for having a followability (or a quick response) of the magnetic fluid to the high-speed relative reciprocations of the two members. In the case of a cylinder/piston device, for example, the reciprocating speed demanded is as high as about 10 mm/s to 30 mm/s. As in the related art, however, only the coating film of the fluorochemical surface modification agent cannot achieve a response to such a high reciprocating speed so that the sealing property cannot be retained.
The present invention has been conceived to solve the aforementioned problems of the related art and has an object to provide a magnetic fluid sealing device, which is intended to prevent the deterioration of the surface treatment and to elongate the lifetime. Another object is to provide a magnetic fluid sealing device, which is excellent in the quick response.
According to the invention, there is provided a magnetic fluid sealing device for sealing the clearance of two members assembled to reciprocate relative to each other, comprising: a magnetic fluid held magnetically on one of the two members and contacting the surface of the other member for sealing the clearance; a groove formed in the surface of the other member for spacing the surface of the other member from the magnetic fluid when the two members are at a specified stop position; and a seal member for sealing the clearance when the two members are at the specified stop position.
In the stopped state of the two members, therefore, the magnetic fluid is positioned in the groove and spaced from the surface of the other member. At the position confronting the magnetic fluid at the stopped time of the two members, therefore, the surface treatment of the other member can be prevented from being deteriorated, but the deterioration of the surface treatment of the surface of the other member does not proceed earlier in a local region so that the lifetime of the device can be elongated.
In the stopped state of the two members, moreover, the seal member seals the clearance between the two members so that the device can keep the satisfactory sealing property.
It is preferable that the edge of the groove for joining the groove and that region of the surface of the other member, which is to be contacted by the magnetic fluid, is chamfered.
As a result, the magnetic fluid can move smoothly from the groove to the region to be contacted by the magnetic fluid, so that the magnetic fluid, which might otherwise be caught by and left on the edge of the groove, can be prevented from being reduced in amount. The magnetic fluid can be held for a long term thereby to elongate the lifetime of the device.
It is preferable that the seal member opens the clearance at the relative reciprocations of the two members.
As a result, at the time of relative reciprocations of the two members, the sliding resistance, as might otherwise be caused by the seal member for sealing the clearance, can be eliminated to lower the driving force necessary for the relative reciprocations of the two members.
It is preferable that the seal member seals the clearance till the magnetic fluid comes into contact with the surface of the other member.
As a result, even when the two members shift from the stopped state to the relatively reciprocating state, the clearance is reliably sealed at all times.
It is preferable that the surface of the other member includes: an outer side region to be contacted by the seal member when the two members take the specified stop position; and an inner side region to be contacted by the magnetic fluid at the time of relative reciprocations of the two members, and that the inner side region is so positioned at a larger distance than the outer side region from the one member that the seal member may be spaced from the surface of the other member at the time of relative reciprocations of the two members.
As a result, at the relatively reciprocating time of the two members, the seal member can open the clearance easily with the driving force applied to the relative reciprocations of the two members.
It is preferable that the surface of the other member is subjected to a wetting-preventing treatment for lowering the wettability with the magnetic fluid.
As a result, the deterioration of the wetting-preventing treatment of the surface of the other member can be prevented at the position confronting the magnetic fluid at the stopped time of the two members, and the deterioration of the wetting-preventing treatment of the surface of the other member does not proceed earlier in a local region, so that the premature wetting of the surface of the other member with the magnetic fluid can be prevented to elongate the lifetime of the device.
It is preferable that the surface of the other member is coated with a surface treating agent having an oil-repelling property.
By thus forming the oil-repelling coating film on the member surface, it is possible to lower the wettability easily with the magnetic fluid.
It is preferable that the surface of the other member is prepared thereon with an organic thin film and is then coated on the organic thin film surface with a surface treating agent having an oil-repelling property.
By thus interposing the organic thin film between the member surface and the oil-repelling coating film (or the coating film of the surface treating agent), the adhesion strength between the member surface and the oil-repelling coating film can be enhanced to prevent the separation or the partial elimination of the oil-repelling coating film thereby to continue the oil-repelling effect (or the wetting-preventing effect) of the member surface for a longer term.
It is preferable that the organic thin film contains polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, poly arylamine, polyvinyl ether or ethylene-vinyl alcohol copolymer, or their mixture or copolymer.
As a result, the sufficient adhesion strength between the member surface and the oil-repelling coating film can be acquired to prevent the separation or the partial elimination of the oil-repelling coating film reliably.
It is preferable that the surface treating agent contains a coupling agent containing fluorine, a monomer containing a fluorine chain, or an oil-repelling agent.
As a result, it is possible to form the oil-repelling coating film, which is excellent in a solvent resistance and a thermal stability.
It is preferable that the surface of the other member is subjected to a surface roughness increasing treatment.
As a result, the surface roughness of the oil-repelling coating film formed on the member surface is increased. By increasing the surface roughness of the coating film surface, moreover, the quick response of the magnetic fluid is raised. Even in case the reciprocating speeds of the two members are high, therefore, the stable sealing property can be kept.
It is preferable that the surface treating agent is mixed with fine particles.
The fine particles in the surface treating agent are dispersed to take a two-dimensional array structure. This two-dimensional array structure is retained even after the coating film was formed, so that a coating film having a high surface roughness can be easily prepared. By raising the surface roughness of the coating film surface, the quick response of the magnetic fluid is raised so that the stable sealing property can be kept even in case the reciprocating speed of the two members is high. Moreover, the member surface need not be subjected in advance to the surface roughness increasing treatment. Therefore, it is possible to facilitate the manufacture and to reduce the cost.
It is preferable that the fine particles are fine particles of silica or polytetrafluoroethylene.
The fine particles of these materials have such weak interactions between them that they can easily take the two-dimensional array structure when mixed with the surface treating agent.